Escaped weeds are keys to resistance

In my last article for Delta Farm Press (Nov. 23, 2007), I began a discussion about the discovery of a Command-resistant population of barnyardgrass in Arkansas. In that article I discussed some ways to get some change or diversity into the pre-emergence grass control program.

For the most part, however, the best opportunity to get more diversity into the weed control program is in the postemergence program. Most weed control programs are still going to begin with Command. In years like last year, when the Command worked so well, I get comments from growers that “all I had to do for grass control was use Command.”

While that is the exception rather than the rule, using Command only for grass control is not always a good thing.

A key to managing resistance with any herbicide is to carefully monitor and study the escaped weeds. Any herbicide can fail for a lot of reasons, and herbicide resistance is way down that list of reasons.

If Command, a pre-emergence herbicide, is not properly activated by moisture, it is going to fail to some degree.

There can be application issues. Perhaps escapes are occurring in areas that were missed; maybe the seedbed was cloddy.

100 percent control with any herbicide is hard to achieve, and each herbicide is better on some weeds than others.

When escapes occur, therefore, they are usually just naturally-occurring and not resistant weeds.

There is the flip side of the issue. It is easy just to assume any escaped weed is naturally occurring or non-resistant. Then, that particular species has escaped for three or four years before you figure out something has to be wrong. By, that time you have a big resistant population.

The subject of escapes is easy to write about in general; it is difficult to be specific. The only way to know if a weed is resistant is to have the seed tested. Obviously, it would not be practical or possible to test all escapes.

Many of the reasons for herbicide failure are obvious. You usually know if the herbicide did not get activated, or if the weeds were drought-stressed when you sprayed them, or if they were too big when you sprayed them. The chances of a weed testing resistant in one of those situations is slim to none.

Likewise, you know the situations where a herbicide has always performed well for you. When you start seeing escapes that you are confident should have been killed, then you need to get aggressive in determining if you have a problem.

If you want to keep Command viable as a pre-emergence herbicide on your farm for years to come, just do not tolerate any escapes. We have a good array of postemergence herbicides to control escaped grasses.

The part that concerns me is barnyardgrass (or related species) resistance to all of our postemergence modes of action has been documented. Propanil and quinclorac resistance has been widely documented in Arkansas and resistance to the modes of action that Ricestar HT, Clincher, Regiment and Newpath belong to have been documented in other rice growing areas.

We can manage these a long time if we use them in combinations and in rotations. I like the combinations of contact and residual herbicides postemergence.

Combinations like propanil or Ricestar HT plus quinclorac following Command are excellent treatments and a lot of growers use them. If you have relied on one of these hard for a couple of years, it may be time to switch to the other one or to use Regiment for escaped barnyardgrass instead.

For a herbicide to be effective, it first has to fit your weed spectrum. However, when there are two or more herbicides with different modes of action that fit the spectrum, look for ways to rotate them.